1. Field of the Invention
The present invention relates to an improved process for the selective hydroformylation of terminal carbons in a terminal olefin group and its use in preparing 1,4-butanedial monoacetal by the reaction of 2-propenal acetal (also known as acrolein acetal or 2-vinyl-5-methyl-1,3-dioxane) with synthesis gas using rhodium complex catalysts and specific organic phosphorus compounds. ##STR2##
2. Discussion of the Background
Hydrolysis and hydrogenation of 1,4-butanedial monoacetal, as shown in U.S. Pat. No. 3,963,754, provides a mixture of 1,4-butanediol and 2-methyl-1,3-propanediol in high yield. ##STR3##
1,4-butanediol is a useful raw material for the production of various organic materials, such as tetrahydrofuran, polybutyleneterephthalate or polyurethanes.
Acrolein is known as a starting material for preparing 1,4-butanediol.
U.S. Pat. Nos. 3,929,915, 3,963,754 and JP-A-Sho 61-39613 report a three-step process for preparing 1,4-butanediol which comprises reaction of acrolein with 2-methyl-1,3-propanediol under conventional conditions to produce acrolein acetal (2-vinyl-5-methyl-1,3-dioxane) (Reaction 1), hydroformylation of the acrolein acetal under conventional conditions to produce the corresponding aldehydes (Reaction 2), and hydrolysis and hydrogenation of the aldehyde of 2-vinyl-5-methyl-1,3-dioxane under conventional conditions to obtain a mixture of 1,4-butanediol and 2-methyl-1,3-propanediol (Reaction 3). ##STR4##
In Reaction 2 above, U.S. Pat. Nos. 3,929,915, 3,963,754, 4,052,401 and 4,003,918 describe that the hydroformylation reaction is carried out in the presence of a rhodium carbonyl complex catalyst Rh.sub.6 (CO).sub.16 with a phosphite ligand having a formula ##STR5## wherein R.sup.1, R.sup.2 and R.sup.3 are the same or different and are C.sub.1 -C.sub.12 alkyl groups or phenyl.
When acrolein acetal is hydroformylated, two isomers, 1,4-butanedial monoacetal and 2-methyl-1,3-propanedial monoacetal, are obtained as schematically shown above in Reaction 2. When 1,4-butanediol is produced through this reaction, the former is the desired compound. From this standpoint, it is important that the hydroformylation reaction is conducted to preferentially obtain the 1,4-butanedial monoacetal in a high yield.
U.S. Pat. No. 3,963,754 discloses that when using trimethyl phosphite as an accelerator of the reaction, a regioselectivity of 80% or more can be achieved with a molar ratio of starting acetal to rhodium catalyst of approximately 1,000. However, the phosphite compound is generally unstable under the hydroformylation reaction conditions (see U.S. Pat. No. 4,717,775). In fact, when trimethyl phosphite is used and a molar ratio of starting acetal to rhodium catalyst is set at approximately 10,000, not only is the prevalent selectivity notably decreased, but trimethyl phosphite is nearly non-detectable in the solution after reaction (see Comparative Example 2 below).
U.S. Pat. Nos. 4,003,918 and 4,052,401 describe that when the hydroformylation reaction is conducted in the presence of a catalyst which is a combination of a rhodium compound and triarylphosphine (wherein the aryl group is a phenyl group or an alkyl-substituted phenyl group) at a molar ratio of rhodium to acrolein acetal of from 1.25.times.10.sup.-5 to 6.2.times.10 .sup.-2, preferably from 1.24.times.10.sup.-4 to 1.24.times.10.sup.-2, 3-formylpropanal acetal is obtained in a yield of approximately 85%. However, the yield thereof is not described in corresponding Examples. In fact, attempts to reproduce their results have shown that the regioselectivity corresponding to the yield described in these U.S. Patents is not reached at all (see Comparative Example 1 below).